1. Field of the Invention
The present invention relates to calibration of the beam position in a charged-particle beam system, such as a scanning electron microscope or electron beam lithography system. More specifically, the invention relates to a method of calibrating deviation of the beam position accompanying a focus correction.
2. Description of Related Art
FIG. 3 schematically shows an electron optical system for use in an electron beam lithography system. This optical system is shown as an example of an electron optical system for use in a charged-particle beam system. In this electron optical system, an image 3 of an object 2 is first formed by a first lens 1. It is conceivable that the object 2 is a shaped apertured plate illuminated with an electron beam (e.g., aperture plate 4 shown in FIG. 1 of U.S. Pat. No. 4,151,422), an electron source, such as an electron gun, or their images.
Then, an image 5 of the image 3 is formed by a second lens 4 on a surface 6 on which a photosensitive material is applied. Under this state, a deflector 7 is operated to deflect an electron beam 8 such that the image 5 is directed at a desired position on the material surface 6. As a result, a desired pattern is written at the desired position on the material surface 6.
The deflection field, i.e., the range deflected by the electron beam 8, is about 1 mm square. A pattern can be written even outside the deflection field by horizontally moving the material stage (not shown) on which the material is carried.
Generally, electromagnetic lenses are used as the first lens 1 and second lens 4. An electrostatic deflector is used as the deflector 7 to provide high deflection speeds.
As the beam is deflected by the deflector 7, deflection aberrations, including comatic aberration, curvature of field aberration (out of focus), astigmatism, distortion aberration (deflection field distortion), and chromatic aberration, are produced.
On the other hand, the height of the material surface 6 is generally not uniform but has a distribution. Furthermore, the height of the material surface 6 relative to the image 5 varies because the material stage moves up and down as this stage moves horizontally. As a result, the image will go out of focus. Also, the beam position on the material surface 6 (i.e., the incident position of the beam on the material surface 6) will deviate from the correct position. These out-of-focus conditions and deviations of the beam position can be treated as aberrations in a broad sense.
Of these aberrations, what are generally corrected by an electron beam lithography system are curvature of field aberration (out of focus), astigmatism, and distortion aberrations (deflection field distortion and beam position deviation). The curvature of field aberration and astigmatism are corrected by electrostatic or electromagnetic correctors. On the other hand, the distortion aberration is corrected by superimposing a correction signal for a positional deviation onto a deflection signal used for positioning.
In the electron optical system shown in FIG. 3, focus correction is made at high speed using an electrostatic focus corrector 9 according to the deflection position. In FIG. 3, none of astigmatic corrector and computing unit for correction of distortion aberration is shown. The focus corrector 9 is made of a cylindrical conductor. When a voltage is applied to the focus corrector 9 relative to a surrounding potential (normally zero potential), the potential distribution in the magnetic field produced by the first lens 1 varies, changing the velocity of electrons passing through the lens 1. This, in turn, varies the intensity of the first lens 1 for electrons. Consequently, the heightwise positions of the images 3 and 5 vary. That is, the heightwise position of the focus relative to the material surface 6 varies.
The aberrations are corrected by the aforementioned method but correction residues (such as overcorrection or undercorrection) are produced. Therefore, it is desired that the aberrations not yet corrected be reduced as much as possible.
One of the causes of correction residues is error produced when aberrations are measured. The method of measuring aberrations is already known. A microscopic pattern of a knife edge or a mesh of a heavy metal is scanned with the electron beam 8. The resulting signal (e.g., a signal based on an absorption current, a signal based on backscattered electrons, or a signal based on secondary electrons) is computationally processed. Thus, the aberrations are measured.
Furthermore, new aberrations produced concomitantly with correction create a cause of generation of correction residues. For example, distortion aberration shows nonlinearity with respect to the deflection voltage. Therefore, if the electron beam 8 is deflected by an amount corresponding to the distortion aberration to correct the beam position, the deflection brings about new distortion aberration, curvature of field aberration, and astigmatism.
In addition, depending on the configuration of the electron optical system, if a focus correction is made, the deflection sensitivity may vary. This can also be treated as a distortion aberration in a broad sense.
Although these correction residues can be made to converge within a certain value by repeating measurement and correction, if aberrations not yet corrected are great, the aberrations are converged slowly. This prolongs the measurement time.
This problem is especially conspicuous in measurement of deflection field distortion for the following reason. The deflection field distortion is corrected by deflecting the electron beam at a large number of points (e.g., 10×10=100 points) in one deflection field and, thus, an exorbitantly long time is taken to make the measurement.
In the system shown in FIG. 3, the object 2 seems to be located on the center axis of the magnetic field set up by the first lens 1. In practice, however, the object 2 often deviates from the center axis of the magnetic field produced by the first lens 1 because the positioning accuracy has a limitation.
Additionally, if the first lens 1 has been produced or assembled at low accuracy or the whole lens is tilted, the center axis of the magnetic field generated by the first lens 1 deviates or tilts. Under these conditions, if the object 2 is located on the proper center axis of the magnetic field, the object 2 actually deviates from the center axis of the field produced by the first lens 1. On the center axis of the magnetic field, electrons traveling through the lens do not vary in velocity. That is, on the center axis, the electron orbit is not affected by the presence or absence of the lens. If the lens is built axisymmetrically, the center axis of the magnetic field is coincident with the axis of axisymmetry.
If a focus correction is made, the projection magnification varies. At this time, if the object 2 deviates from the center axis of the magnetic field produced by the first lens 1, the beam position is made to deviate by a variation in the projection magnification. That is, the position on the material surface 6 at which the beam impinges varies. The beam position deviation is added to distortion aberration, increasing the deflection field distortion.
Accordingly, it is desired that the deflection field distortion be reduced before measurement by previously measuring beam position deviation accompanying a focus correction and causing the beam position to be corrected according to the amount of focus correction.
However, apparently nothing is known as to how a beam position deviation accompanying a focus correction should be measured. Furthermore, apparently nothing is known as to how the amount of correction to the deviation should be determined.